1. Field of the Invention
The present invention relates to a process of drying a particulate material, in which a gas is passed through the particulate material. The particulate material is held at least during a part of the drying process at a temperature, at which at least one part of the substance to be extracted from it is in the solid state. The invention also relates to an apparatus for drying a particulate material. This apparatus comprises a container to receive the particulate material to be dried, and means to pass the gas through the particulate material. At least one cooling means is provided for cooling the particulate material.
2. Description of the Prior Art
Sublimation drying processes are known in the art. In a process of this kind a material to be dried, or more accurately, the water contained in the material and intended to be extracted from it during drying, is frozen before the drying operation and then transformed into vapor by sublimation. The material is usually dried in batches in evacuated containers, in which it rests motionless on a support surface. The heat required for sublimation is supplied to the material either by way of a contact surface, through heat conduction, or by radiation. The vapors developing during the drying are usually frozen on cold surfaces, or sorbed by some means of sorption, or sucked off by steam ejector pumps.
In as much as a particulate material present in an evacuated space possesses but low heat conductivity and since furthermore the vapor generated during the drying process can leave the material only with relative slowness from a material resting motionless on a support surface, there follows that a layer of such a material resting on said support surface cannot be but comparatively thin, to avoid excessive slowness in the progress of the drying process. Thus, only relatively small quantities of the material may be dried per batch in a container. In order to avoid any melting of the ice at the boundary surface of the particulate material at which the latter receives heat by conduction or radiation, it is necessary to keep the quantity of heat supplied per unit of time very small because of the low heat conductivity of the particulate material. The time requirements for drying thus become very high, even if the batches and the layer thicknesses of the material are low.
It is further known to have a material to be dried in a stationary container subjected to whirling by means of a current of warm air, so that the whirling layer is formed by the material. It is also known to dry a material in a drum-shaped rotated container provided with a perforated wall, by passing warm air through the particle bed provided in the container and through the zone of the perforated wall covered by said particle bed. Attention is called in this connection to the International Disclosure Publication WO 82/03972 and corresponding U.S. Pat. No. 4,543,906, as well as U.S. Pat. No. 4,476,804. It is also known to move the material within a stationary container by means of a movable member engaging the material, while air or nitrogen is passed through the material and the container wall is heated. All these processes in which the material is heated relatively intensely can have disadvantages when drying materials consisting of thermolabile substances or having a porous structure to be left unchanged during drying.